1. Field of the Invention
The present invention relates to a vehicle control device that controls the vehicle driving force and vehicle braking force to be applied to a vehicle to maintain the vehicle body speed (vehicle speed) at a target vehicle speed.
2. Description of the Related Art
There have been previously proposed vehicle control devices (also referred to as constant speed traveling control devices or automatic traveling control devices) for controlling the vehicle driving force and vehicle braking force to be applied to a vehicle to maintain the vehicle speed at a relatively low target vehicle speed of, for example, about several kilometers per hour to a dozen kilometers per hour (at as low a target vehicle speed as the creep speed) (see Japanese Patent Application Publication No. 2004-90679 (JP-A-2004-90679), for example). Such control is also referred to as “constant speed traveling control.” Since such a control device changes not only the vehicle driving force but also the vehicle braking force to control the vehicle speed, the control device can control the vehicle speed quickly and accurately as compared to the case in which only the vehicle driving force is changed. As a result, the control device can make the vehicle speed accurately equal to the target vehicle speed even when the vehicle is traveling on a steep grade (uphill or downhill grade).
The control device of the related art changes the vehicle driving force toward a vehicle driving force corresponding to the amount of accelerating operation (for example, the accelerator displacement) and changes the vehicle braking force toward a vehicle braking force corresponding to the amount of braking operation (for example, brake pedal depression amount) after the constant speed traveling control is terminated. Such a control device may have a switch for constant speed traveling control, and may be configured to perform the constant speed traveling control when the switch for constant speed traveling control is on and terminate the constant speed traveling control when the switch for constant speed traveling control is turned off. Thus, the control device constituted as described above reduces the vehicle driving force toward “0” at a predetermined driving force reduction rate and reduces the vehicle braking force toward “0” at a predetermined braking force reduction rate when the switch is operated and termination conditions of the constant speed traveling control are satisfied while neither an accelerating operation nor a braking operation is performed.
However, when the control device reduces the vehicle driving force and the vehicle braking force toward “0,” the control device does not associate the vehicle driving force and the vehicle braking force with each other. In other words, the driving force reduction rate of the vehicle driving force and the braking force reduction rate of the vehicle braking force are independent of and different from each other. Therefore, the control device may cause the vehicle to accelerate or decelerate while the vehicle driving force and the vehicle braking force are decreasing toward “0” after the termination of constant speed traveling control as described below and, consequently, give the driver discomfort.
For example, when the switch is operated and termination conditions of the constant speed traveling control are satisfied during constant speed traveling control, and when the vehicle braking force decreases more quickly than the vehicle driving force, the vehicle driving force becomes excessive relative to the vehicle braking force. As a result, since the vehicle accelerates even when the vehicle is traveling on an uphill grade, the driver may feel discomfort. Also, when the switch is operated and the termination conditions of the constant speed traveling control are satisfied during constant speed traveling control, and when the vehicle driving force decreases more quickly than the vehicle braking force, the vehicle braking force becomes excessive relative to the vehicle driving force. As a result, since the vehicle decelerates even when the vehicle is traveling on a downhill grade, the driver may feel discomfort. Such a situation occurs not only after constant speed traveling control is terminated by an operation of the switch for constant speed traveling control but also after constant speed traveling control is terminated because some part of the control device is determined not to be working properly, for example.